Direct and indirect drift assessment means, Part 2: Wind tunnel experiments

Wind tunnel measurements, performed in Silsoe Research Institute (SRI), were used to measure airborne and fallout spray volumes under directly comparable and repeatable conditions for single and static nozzles. Based on these measurements, drift potential reduction percentages (DPRP), expressing the percentage reduction of the drift potential compared with the reference spraying, were calculated following three approaches. The first approach was based on the calculation of the first moment of the airborne spray profile (DPRP_v1). In the second and third approach, the surface under the measured airborne (DPRP_v2) and fallout (DPRP_H) deposit curve were used. These DPRP values express the percentage reduction of the drift potential compared with the reference spraying. Ten different spray nozzles were tested. The results showed the expected fallout profiles with the highest deposits closest to the nozzle and a systematic decrease with distance from the nozzle. For the airborne deposit profiles, the highest deposits were found at the Lowest collectors with an important systematic decrease with increasing heights. For the same nozzle size and spray pressure, DPRP values are generally higher for the air inclusion nozzles followed by the low-drift nozzles and the standard flat fan nozzles and the effect of nozzle type is most important for smaller nozzle sizes. In general, the bigger the ISO nozzle size, the higher the DPRP values. Comparing results from the three different approaches namely, DPRP_v1, DPRP_v2 and DPRP_H, some interesting conclusions can be drawn. For the standard flat fan nozzles, DPRP_v1, values were the highest followed by DPRP_v2 and DPRP_H while for the low-drift nozzles opposite results were found. For the air inclusion nozzles, there was a relatively good agreement between DPRP_v1, DPRP_v1 and DPRP_H values. All of this is important in the interpretation of wind tunnel data for different nozzle types and sampling methodologies.